This invention relates generally to X-ray tube anode targets, and more specifically to brazed X-ray tube anode targets.
X-ray beam generating devices, or X-ray tubes, typically comprise dual electrodes of an electrical circuit within an evacuated chamber or tube. The electrical circuit generates a beam of electrons directed toward an anode target. A surface of the anode target converts the kinetic energy of the electron beam against the target to high frequency electromagnetic waves, i.e., X-rays, which are collimated and focused for penetration through an object for internal examination purposes.
The high velocity electron beam impinging on the target surface, or focal track, generates extremely high and localized temperatures in the target structure accompanied by high internal stresses leading to deterioration and breakdown of the target. Consequently, a rotating anode target is typically used to minimize localized heat concentration and stresses. By rotating the target, a focal track region impinged by the electron beam is continually changed and the heat effects are better distributed throughout the structure. See, for example, U.S. Pat. No. 5,414,748.
One type of known rotating anode target includes a refractory metal cap having a focal track for producing X-rays when bombarded by the electrons from a cathode according to known techniques. A graphite back is attached to the cap by known brazing methods to provide a heat sink for the heat which is transferred from the metal cap and from the focal track. See, for example, U.S. Pat. No. 5,178,136. However, during extended operation of an X-ray tube, separation of the brazed graphite back from the metal cap has been observed as an end of life failure mode.
Accordingly, it would be desirable to provide a longer life X-ray target that avoids the failure mode of separation of the graphite back and cap.
In an exemplary embodiment of the invention, a rotatable X-ray target includes a circular cap having an outer edge and a stepped surface adjacent the outer edge. A focal track is formed on a first surface of the cap adjacent the outer edge. A step extends radially inward from the outer edge and a graphite back is brazed to the step. A corner of the step is moved radially inward from the cap outer edge, thereby distancing the corner from the focal track where the maximum heat is generated and reducing a heat load on the corner. The graphite back extends radially outward beyond the step, thereby reducing the thermal stress in the graphite and increasing a thermal storage of the graphite.
A recess is formed into the cap first surface between the focal track and a rotational axis to maintain a selected moment of inertia of the target and thereby maintain the rotor dynamics of a given X-ray tube. Consequently, the brazed step joint encounters less heat and reduces the strain on the braze material, thereby reducing instances of separation of the brazed graphite back.